Water absorbent resins are widely utilized in a variety of applications, such as hygiene products such as disposable diapers, sanitary napkins, and incontinence products for adults, and soil water retention agents, and thus, water absorbent resins are produced and consumed in large quantities. Known examples of such water absorbent resins include a partially neutralized crosslinked product of polyacrylic acid; a hydrolysate of a starch-acrylic acid graft polymer; a saponification product of a vinyl acetate-acrylic acid ester copolymer; a hydrolysate of an acrylonitrile copolymer or an acrylamide copolymer, or a crosslinked product thereof; and a crosslinked product of a cationic monomer, and the like. These water absorbent resins are produced by, for example, a method of polymerizing an aqueous solution containing a hydrophilic monomer while pulverizing the polymer gel by stirring (Patent Literature 1), methods of static polymerizing an aqueous solution containing a monomer (Patent Literatures 2 to 6), reverse phase suspension polymerization, or dropping polymerization, and the like.
In recent years, thin type hygiene products with smaller contents of cotton-like pulp are commercially available; however, the use amount of water absorbent resins has increased accordingly, so that there is a demand for a further improvement of the physical properties of water absorbent resins. Under such circumstances, an increase in the water absorption rate of water absorbent resins is desired, and a wide variety of technologies have been suggested by various water absorbent resin manufacturers (Patent Literatures 7 to 33). For example, a method of dispersing a large amount of gas bubbles in an aqueous monomer solution, and using a large amount of a surfactant, or optionally a gas bubble stabilizer, so as to prevent the disappearance of the gas bubbles until the initiation of polymerization (Patent Literatures 8, 20 and 21); a method of maintaining an aqueous monomer solution at a low temperature (Patent Literatures 7 and 9); and a method of producing a foam-like water absorbent resin (Patent Literatures 8, 20, 21, and 30 to 33) have been suggested.
Furthermore, foaming polymerization is carried out for the purpose of enhancing the water absorption rate through an increase in the surface area, and in regard to this foaming polymerization, specifically, as a foaming agent that is used for the monomer, a technology of using a carbonic acid salt (Patent Literatures 10 to 17); a technology of using an organic solvent (Patent Literatures 18 and 19); a technology of using an inert gas (Patent Literature 22); a technology of using an azo compound (Patent Literatures 23 and 24); a technology of using an insoluble inorganic powder (Patent Literature 25), and the like are known. Furthermore, a technology of carrying out foam formation and crosslinking after polymerization (Patent Literature 26), a technology of using water-insoluble particles in the polymerization (Patent Literature 27), a technology of performing polymerization in a state in which precipitates of an acrylic acid sodium salt are dispersed (Patent Literature 28), and the like have been proposed.
Furthermore, in the method of producing a foam-like water absorbent resin, the purport of recommending “avoidance of boiling at the time of polymerization” (Patent Literatures 8, 20, 21, and 30 to 32) has been disclosed, or “polymerization at or below 65° C.” (Patent Literature 33) has been disclosed.
In addition, in regard to polymerization other than foaming polymerization, a technology of controlling the maximum temperature to be low for the purpose of reducing a soluble fraction or the like has been disclosed. Specifically, a technology of adjusting the maximum temperature to 95° C. or lower (Patent Literature 34), a technology of performing polymerization at a polymerization temperature of 20° C. to 70° C. (Patent Literature 35), and a technology of performing polymerization at a polymerization temperature of 20° C. to 95° C. (Patent Literature 36) have been proposed.
However, in the method of dispersing a large amount of gas bubbles by using a large amount of a surfactant, at least continuous gas bubbles are formed by polymerization, but a relatively long time is required for the dispersion of gas bubbles. Also, when a water absorbent resin produced by that method is used in hygiene products, the interface (surface) tension of the body fluid is decreased by the large amount of surfactant, and therefore, there is a problem that the liquid absorption characteristics of the hygiene products are impaired, and the amount of re-wetting increases. Furthermore, in the method of maintaining an aqueous monomer solution at a low temperature, since a long time is required for polymerization, productivity becomes poor, and also, there is a problem that there is almost no interconnection of gas bubbles (continuous gas bubbles) in the obtained porous polymer, and the extent of increase in the water absorption rate becomes low in spite of the decrease in the bulk density. Furthermore, in the step of foaming polymerization by which polymerization of a monomer is carried out, with gas bubbles dispersed in the monomer, which has been suggested in the past in the Patent Literatures described above and the like, an excessive decrease in the bulk density occurs, so that the transportation cost and the storage cost are increased. Also, there are also problems of deterioration of physical properties caused by a decrease in the impact resistance of the water absorbent resin powder, and powder dust generation.
As discussed above, many methods for increasing the water absorption rate have been proposed as in Patent Literatures 7 to 33 and the like; however, these methods require expensive production facilities, exhibit low productivity, or are accompanied by an increase in the cost due to the use of large amounts of surfactants, or by a decrease in the surface tension of the water absorbent resin (and a consequent increase in the amount of re-wetting of diapers). In addition, these methods still bring about insufficient formation of foams, and further foaming causes impairment of the water absorption characteristics (for example, water absorption capacity under load, liquid permeability, extractable polymer content, and residual monomer), or, since the volumetric expansion factor caused by foaming is high, the bulk density is excessively decreased, and thereby, fine powders increase. Furthermore, the “avoidance of boiling at the time of polymerization” that is recommended in Patent Literatures 8, 20, 21 and 30 to 33, and the like causes a decrease in productivity or requires expensive cooling facilities and the like.
Furthermore, since water absorbent resins are generally in the form of powder (particulate form), when the water absorbent resins are actually used in disposable diapers, sanitary napkins, tampons, and the like, it is necessary to manufacture the powdered (particulate) water absorbent resin into a sheet form, a cylindrical form or the like after mixing a water absorbent resin with a fibrous material or an adhesive as necessary. Such manufacturing process causes not only an increase in the cost, but also a decrease in the water absorption rate or powder destruction (generation of fine powder).
Also, in addition to the problem of the water absorption rate, water absorbent resins are mainly used in hygiene materials, and along with the increase in the use amount (g or % by weight) per one hygiene material, the degree of whiteness of the water absorbent resin itself is more emphasized, so that the problem of coloration is also increasing. Thus, Patent Literature 29 discloses a method for preventing coloration by using an acrylic acid ammonium salt in the monomer, or a method of preventing coloration by adding a compound containing a phosphorus atom or a sulfur-based reducing agent multiple times.
Furthermore, while water-containing gel-like crosslinked polymers obtainable by polymerizing an aqueous monomer solution require large facilities or a large amount of heat energy, and a long drying time for drying of the polymer, there are also problems such as deterioration or coloration of the water absorbent resins caused by drying for a long time, and an increase in the production cost for the water absorbent resins. Today, shortening of the drying time is an important problem to be addressed for the water absorbent resins, from the viewpoints of cost, physical properties, and coloration.
Also, while conventional water absorbent resins are primarily in the form of powder, in the case of using the water absorbent resins as absorbing articles such as disposable diapers (final consumption materials), there is a problem that a step of fixing or incorporating a water absorbent resin powder into an absorbing article may be needed, an expensive adhesive for the fixation should be used, and powder may be dropped-out or migrate.
In order to solve such problems with powder, a water absorbent resin molded product in the form of sheet etc. or a composite have also been suggested (Patent Literatures 37 and 38). However, complicated steps or expensive auxiliary raw materials are needed for molding, and also, since molded products such as a sheet form have greatly decreased specific surface areas as compared with powders, such molded products have insufficient water absorption rate and insufficient gas permeability, and cause leakage or stuffiness of disposable diapers and the like.